(cba:news) observing techniques
jop at astro.columbia.edu
Thu Jul 1 17:53:57 EDT 2004
By the way, I wanted to stress again the advisability of what has become
our standard observing technique: white-light photometry with good time
resolution (better than ~60 s). Unless you have a 20-inch or better, you
just can't afford to lose the factor of 8 in counts that a filter will
exact. With rare and quite subtle exceptions, the waveforms of CVs very
closely resemble each other in all passbands relevant to ground-based
photometry. So with filters you degrade the signal and get little back in
return. The best reason to use filters is not that you'll see new
phenomena (because you won't - or worse, you will because your data is now
of much lower quality)... but because it minimizes differential
extinction, which is a hazard of our enterprise.
With the 1.3 m telescope on Kitt Peak, we usually use a V filter, but only
to reduce differential extinction - and only because we have enough
photons to reject 90% of them. Occasionally for a special purpose we'll
spin filters and get BVRI time series. But only in specialized cases.
For faint stars the penalty is really huge - a factor of 8 in counts, and
about a factor of 6 in time resolution. At 10th magnitude you might be
able to afford that... but generally your data will be of much greater
usefulness if you observe unfiltered (and fairly fast).
Nothing new there; just a reminder.
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